Theoretical Studies on Structures and Electronic State of Alkyl-Substituted Ethyl Cations

Toshiaki Shimasaki,Tatsuaki Yamaguchi,Mitsuhiro Shibata,Ken Nakayasu

doi:10.1155/2013/406904

Toshiaki Shimasaki, Tatsuaki Yamaguchi + Show 2 more

Open Access

https://doi.org/10.1155/2013/406904

Copy DOI

Abstract

This paper describes the gas phase structural details for a series of alkyl-substituted ethyl cations 1–11 by mainly natural bond orbital (NBO) analysis. For the simplest ethyl cation, two kinds of optimized structures (nonclassical 1 and classical 2) were found to have minima on the potential surface, and the nonclassical 1 was more stable than the corresponding classical 2 by 1.19 kcal/moL at Moller-Plesset (MP) level calculations in the gas phase. The calculated values of Wiberg’s bond index revealed that the central ethylic C1–C2 bonds of the nonclassical cations 1, 6, 7, and 10 have 1.10–1.46 bond multiplicities. The compliance constant () values indicated that the cation center C1 of nonclassical 2-butyl cation 6 (0.209 A/mdyn) acquires stronger hyperconjugative stabilization from C2–R3 (C–H) bond than that of the nonclassical ethyl cation 1 (0.388 A/mdyn). Moreover, the gas phase stabilities for the alkyl-substituted ethyl cations 1–11 were also estimated by the hydride affinities based on isodesmic equations.

Highlights

The carbocations are important chemical species as intermediates in various organic reactions and have been gathering much attention on their structure and electronic state [1]
The SCF energy, Wiberg’s bond index, and compliance constant calculation for each optimized ethyl cation were verified using the same basis set, and their results were summarized in Tables 3 and 4
Gas phase structural and electronic properties of alkyl-substituted ethyl cations 1–11 were investigated at MP2 level

Summary

Introduction

The carbocations are important chemical species as intermediates in various organic reactions and have been gathering much attention on their structure and electronic state [1]. Yannoni et al have studied the alkyl cation and especially revealed the structural details of tert-butyl cation (1,1-dimethylethyl cation) by NMR methods in 1989 [3]. Farcasiu et al have theoretically investigated the conformations of carbocations and the reactions of their precursors with borane at high-level ab initio calculations [4, 5]. In 2007, Ricks and coworkers revealed that extraordinary stability of C4H9 + species is brought about the charge distribution and hyperconjugation by means of IR spectral analysis [6]. Feng et al reported the structural properties and energetics at high-level MO calculations on tert-butyl cation [7]

Methods

Results

Conclusion